Combined Sewer Overflow
Measures the number of combined sewer overflow locations across the country within how many municipalities, and the volume of untreated sewage coming from these sources.

Above image: Impacts of CSOs into the Toronto Harbour at 3:26pm July 17th, 2018, following 8 mm of rain. Captured from the CN tower EarthCam by Swim Drink Fish.


Untreated sewage can end up in our waterways in multiple ways. Combined sewer overflows (CSOs) are one major source, where pipes that carry wastewater from our houses and businesses also carry stormwater to treatment plants. In heavy rainstorms, more of this combined water than the treatment plant can handle accumulates, activating overflow points that redirect untreated water into our waterways- our rivers, lakes and ocean.

The OLW Network is tracking this measure in three ways: how many CSO locations exist across the country, how many municipalities have them, and what the volume of the untreated sewage entering our waterways is.

We know that any of these measures on their own have limitations: measuring the # of CSOs and their locations only disguises the knowledge that some CSO locations overflow more than others, in addition to some technological solutions that reduce CSO overflow events such as real time controls. Measuring volume (at this time) also presents some problems: the reliability of volume data is not consistent across the country, with some discharge locations reporting based on models not actual measurements, and others having noticeable gaps in the reporting. However, on the positive side, as of June, 2019 Statistics Canada has released the data they do have on CSO discharge volumes across the country.

Tracking all three of these metrics however gives us a more complete picture, while allowing us to focus on a suite of solutions, from reducing the total number of CSO locations across the country as well as pursuing other solutions to reduce the amount of untreated sewage that comes from these sources.

In 2015, a reported 1,516 combined sewer overflow locations existed across 130 municipal systems, and according to Statistics Canada that year, 168,800,000 cubic metres of untreated sewage was released from CSOs into the water. Please note that the actual volume of untreated sewage released into Canadian waters is likely higher than reported. Across Canada 268 municipal systems are required to report CSO discharges, however many do not report accurate data concerning the volume and rate of sewage releases to Environment Canada. For example: in 2015, only 36 of 168 municipal systems in the province of Quebec filed completed combined sewer overflow reports; some municipal systems filed a record of overflow events but did not report any volumes; the City of Toronto did not report overflow events or volumes for January, February, March, November, or December.  

To request a copy of CSO overflow data for your community, please email OLW Network member Swim Drink Fish at: [email protected]

5-Year target: To see a reduction in any of the three metrics. The ultimate goal is for no untreated sewage to enter our waterways and for CSOs to be replaced across the whole country. If you or your organization wants to tackle combined sewage pollution in your community, Our Living Waters and Ottawa Riverkeeper published this helpful toolkit for Community Action in 2018, titled Tackling Combined Sewer Overflows.

Last updated August 2019

Our Living Waters Network Members leading this conversation:

If you would like to be part of this conversation, email us at [email protected].

Note: The data presented here represents our best research given the time and resources at hand. We acknowledge there may be errors. This shared measurement system belongs to all members of the Our Living Water Network, so if you have any corrections for us, or ideas to share on this measure, please send us an email at [email protected].

Untreated Wastewater|Measures the percentage of wastewater that goes untreated across Canada.
Combined Sewer Overflow|Measures the number of combined sewer overflow locations across the country within how many municipalities, and the volume of untreated sewage coming from these sources.
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